The field of the present invention is servo apparatus and methods. More particularly, the present invention relates to servo apparatus and methods of the type used to control or direct the course of a dirigible projectile or automotive vehicle, such as a torpedo or airborne missile. In such an application the servo apparatus is generally known as a fin actuator or control surface actuator, and is employed to move and selectively position a fin, rudder, or other movable control portion of the vehicle to effect the course of movement thereof in response to a control signal.
With still greater particularity, the present invention relates to novel valve apparatus having particular utility in servo apparatus of the above-described character. The inventive valve apparatus herein set forth may be employed to control a flow of pressurized compressible or incompressible fluid from a source to a first receiver and from the first receiver to a lower pressure second receiver in response to respective input signals or commands.
When employed in a servo system of the above described character, the first receiver is a fluid pressure responsive motor driving the vehicle control portion, while the second receiver is a low pressure sump or vent to ambient pressure. The valve apparatus is operatively associated with, or integrally formed with, a transducing device which drives a portion of the valve apparatus between selected operative positions in response to input signals. By way of example, the input signals may be electrical voltage or current signals originating with a vehicle autopilot. In such a case, the transducing device may take the form of a solenoid coil, or of a Galvenometer movement coupled to the valve apparatus via force amplifying structure.
A conventional servo valve is known in accordance with U.S. Pat. No. 4,151,856 granted May 1, 1979 to B. S. Burrus, wherein an input member provides an input signal by moving an input portion of the servo valve to a selected position. The input portion of the servo valve cooperates with a plug member and a movable wall to define a two-way pilot valve. Communication of pressure fluid is controlled by the pilot valve to and from a force amplifying diaphram actuating a second or booster two-way valve.
With a servo valve according to the invention of Burrus, it is believed the input signal must overcome substantially unbalanced forces originating both with fluid pressures and with resilient members within the valve before the pilot valve is moved from a closed to an open position to effect an increase or decrease of output pressure. Consequently, the servo valve of Burrus is believed to have an inherent force dead band wherein forces exerted by the input member do not effect an output pressure change. The input member forces must, it is believed, exceed the force dead band and effect a movement of the input member before an output pressure change can be effected.
Further, with respect to the servo valve of Burrus, the gain of the valve, that is, the ratio of output pressure change to input signal change, is believed to be substantially constant regardless of the rate or frequency of input signal changes applied to the servo valve. Such a substantially constant servo valve gain within a broad frequency band width may present no serious or unacceptable detriment in a relatively low fidelity servo system. In such a servo system, the output response of the system may permissively deviate from that commanded by the input signal, particularly during transient response of the system.
On the other hand, because a narrow band width servo system of relatively high fidelity needs respond to input signals only within a relatively narrow range of frequency, the servo system can be designed to be stable and provide a response of high fidelity within the permissible frequency band for the input signal, even with a servo valve like that of Burrus. However, when a conventional servo valve, like that of Burrus, is employed in a servo system required to provide high fidelity over a broad frequency band width, or rate of input signal change, many deficiencies become apparent.
By way of example only, the mission requirements of a very high-speed dirigible automotive vehicle, such as an air-to-air missile, well illustrate a very challenging problem to a servo system. On one hand, missile launching and terminal phases of the mission require large, high-powered, frequent excursions of the missile aerodynamic control surfaces. These large excursions will be made in response to correlative input signals, and must be well damped and executed with stability and without overshoot. Such large excursions of the control surfaces are necessary to achieve launching of the missile while avoiding collision with the parent aircraft, and to successfully follow an evasive target.
On the other hand, cruising phases of the missile mission require only comparatively small and infrequent control surface movements for course correction. However, these small course-corrective movements must in fact be made by the servo system in response to small input signals. That is, if the small input signals are lost to servo system dead band or hysterisis, a course correction will be effected only after the course error has reached a higher level. The missile will then undesirably follow a wandering course.
In view of the above, it will be appreciated that each phase of operation of a high-speed dirigible automotive vehicle, of whatever type, requires differing characteristics of the control servo system. Launch and terminal mission phases require a low-gain, high powered servo system which is stable at high frequencies. Missile cruise, on the other hand, requires a high-gain, sensitive and responsive servo system which will effect small input signals.
However, it is believed conventional servo apparatus cannot satisfactorily fulfill the full range of requirements set out above. As pointed out above, conventional servo valves generally have a substantially flat gain curve as a function of input frequency. However, the control surface movement requirements outlined above, and other servo system applications, require a relatively high system gain at low frequencies and a lessening gain as a function of input signal frequency. This requirement is not necessarily within the scope of conventional servo apparatus and servo valves.